1. Field of the Invention
The present invention relates to a thin film transistor, a method of manufacturing the same, and a flat panel display having the thin film transistor.
2. Description of the Related Art
Thin film transistors (TFTs) are used for flat panel displays such as liquid crystal displays or organic light emitting diode (OLED) displays.
FIG. 1 shows a driving circuit of a unit pixel of a conventional flat panel display. As depicted in FIG. 1, two TFTs 10 and 12 and one capacitor 14 are used for driving each pixel. The first TFT 10 is a switching device and the second TFT 12 is a driving device. The first and second TFTs 10 and 12 have the same configuration.
FIG. 2 is a cross-sectional view of a configuration of the second TFT 12 when the second TFT 12 has a bottom gate structure.
Referring to FIG. 2, a gate 22 is formed on a substrate 20, and a gate insulating layer 24 covering the gate 22 is formed on the substrate 20. A channel layer 26 is formed above the gate 22. The channel layer 26 is located above the gate 22. The channel layer 26 is an amorphous silicon layer or a poly silicon layer. A source 28 and a drain 30, which contact the channel layer 26, are formed on the gate insulating layer 24. The source and drain 28 and 30 are metal layers, separated from each other, and contact different ends of the channel layer 26.
The second TFT 12 must have high stability so as not to be affected by current stress and have uniform device characteristics over the entire back plane of the display. The characteristics of the TFT are closely related to the channel layer 16.
If the channel layer of the second TFT 12 is an amorphous silicon layer, the second TFT 12 can have uniform device characteristics. In this case, each of the pixels can display an identical color on the entire region of the display. However, the color is very sensitive to current stress. Thus, if the second TFT 12 is operated for a long period of time, a threshold voltage Vth of the second TFT 12 increases. FIG. 3 shows an example of the increase in the threshold voltage Vth of a TFT.
More specifically, FIG. 3 is a graph showing the variation of the threshold voltage Vth of a TFT, in which the channel layer is an amorphous layer, according to the driving time. The variation of the threshold voltage Vth was measured by operating the TFT for 100 hours while maintaining the TFT at a temperature of 50° C.
Referring to FIG. 3, the threshold voltage Vth of the TFT in which the channel layer is an amorphous layer was increased to approximately 2.1 V after 100 hours of operation.
If the channel layer of the second TFT 12 is a poly silicon layer, the second TFT 12 is affected little by the current stress, however, the uniform device characteristics are reduced when compared to the case that the channel layer is an amorphous layer.
The device characteristics of the TFT when the channel layer is formed of amorphous silicon or poly silicon can be improved to some extent by configuring a compensation circuit in each of the pixels. However, even though the compensation circuit is configured, the improvement of the device characteristics is limited. Also, since the compensation circuit includes a few TFTs and a few capacitors, the number of devices is increased greater than the case depicted in FIG. 1. Thus, the manufacturing process becomes complicated, thereby increasing the manufacturing costs.